WO2001003886A1

WO2001003886A1 - Method and apparatus for eliminating wear and grooving of workpiece carrier retaining element

Info

Publication number: WO2001003886A1
Application number: PCT/US2000/018303
Authority: WO
Inventors: Wayne F. Lougher
Original assignee: Speedfam-Ipec Corporation
Priority date: 1999-07-09
Filing date: 2000-06-30
Publication date: 2001-01-18

Abstract

An improved workpiece carrier retaining structure (100) includes at least one element for retaining a workpiece that is comprised of a hard ceramic material. The retaining element may take the form of a retaining ring (110) comprised of a hard ceramic material which optionally includes a disposable liner (112), preferably made of a softer material than the workpiece (120), which prevents the retaining element (110) from contacting the workpiece (120) during polishing. The ceramic material comprising the retaining element (110) may include more than a single-phase ceramic. For example, the retaining element (110) may be comprised of one ceramic material that is reinforced with another ceramic material. Forming a workpiece retaining element (110) from a hard ceramic material results in eliminating or reducing wear and grooving of the retaining element (110) during polishing.

Description

METHOD AND APPARATUS FOR ELIMINATING WEAR AND GROOVING OF WORKPIECE CARRIER RETAINING ELEMENT

FIELD OF THE INVENTION The present invention relates, generally, to systems for polishing or planarizing workpieces such as semiconductor wafers. More particularly, the present invention relates to a method and apparatus for eliminating wear and grooving of a workpiece carrier retaining element where the workpiece carrier engages a workpiece against a polishing surface during a polishing procedure.

BACKGROUND OF THE INVENTION Many electronic and computer-related products such as semiconductors, CD-ROMs, and computer hard disks, require highly polished surfaces in order to achieve optimum operational characteristics. For example, high-quality and extremely precise wafer surfaces are often needed during the production of semiconductor-based integrated circuits. During the fabrication process, the wafers generally undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures created on the wafer surface. As the size of integrated circuits decreases and the density of microstructures on integrated circuits increases, the need for accurate and precise wafer surface polishing increases.

Chemical Mechanical Polishing ("CMP") machines have been developed to polish or planarize semiconductor wafer surfaces to the flat condition desired for integrated circuit components and the like. For examples of conventional CMP processes and machines, see U.S. Pat. No. 4,805,348, issued February 21, 1989 to Arai et al.; U.S. Pat. No. 4,811,522, issued March 14, 1989 to Gill; U.S. Pat. No. 5,099,614, issued March 31, 1992 to Arai et al.; U.S. Pat. No. 5,329,732, issued July 19, 1994 to Karlsrud et al.; U.S. Pat. No. 5,498,196, issued March 12, 1996 to Karlsrud et al.; U.S. Pat. No. 5,498,199, issued March 12, 1996 to Karlsrud et al.; U.S. Pat. No. 5,558,568 issued September 24, 1996 to Talieh et al.; and U.S. Pat. No. 5,584,751, issued December 17, 1996 to Kobayashi et al.

Typically, a CMP machine includes a wafer carrier configured to hold, rotate, and transport a wafer during the process of polishing or planarizing the wafer. The wafer carrier is rotated to cause relative lateral motion between the polishing surface and the wafer to produce a substantially uniform thickness. In general, the polishing surface includes a horizontal polishing pad that has an exposed abrasive surface of cerium oxide, aluminum oxide, fumed/precipitated silica, or other particulate abrasives. Commercially available polishing pads may utilize various materials, as is known in the art. Typically, polishing pads may be formed from a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Arizona. The hardness and density of the polishing pad depends on the material that is to be polished and the degree of precision required in the polishing process.

During a polishing operation, a pressure applying element (e.g., a rigid plate, a bladder assembly, or the like), which may be integral to the wafer carrier, applies pressure such that the wafer engages the polishing surface with a desired amount of force. The carrier and the polishing pad are rotated, typically at different rotational velocities, to cause relative lateral motion between the polishing pad and the wafer and to promote uniform polishing. Most conventional carrier assemblies include some form of retaining structure that maintains the position of the wafer under the pressure element during polishing. Prior art carrier assemblies designed for compatibility with circular wafers employ round retaining structures such as retaining rings.

Retaining rings may either be fixed or "floating" within the wafer carrier. For example, U.S. Pat. No. 5,695,392, issued December 9, 1997 to Kim, discloses the use of a fixed retaining ring collar that is bolted to the main carrier housing. U.S. Pat. No. 5,584,751, issued December 17, 1996 to Kobayashi et al., and U.S. Pat. No. 5,795,215, issued August 18, 1998 to Guthrie et al., each teach the use of a floating retaining ring and a pressure regulating mechanism that controls the biasing pressure applied to the retaining ring.

Typically, retaining rings are made from engineering polymers such as, for example, acetal homopolymer, acetal copolymer, and polyphenylene sulfide. These materials are prone to wear due to the friction between the wafer, polishing pad and slurry abrasives that are used during polishing of the wafer. Wearing of the materials that comprise the retaining rings results in shortening the lives of the retaining rings which are a functional and necessary component of the wafer carriers. Water absorption by the retaining rings can also distort dimensions of the acetal copolymers or homopolymers which comprise the retaining rings thereby distorting the dimensions of the retaining rings themselves. Downtime associated with the repair or replacement of wafer retaining elements such as, for example, retaining rings, used in wafer carriers may be extremely undesirable, particularly if the workpiece throughput rate is critical.

Accordingly, there is a need for a method and apparatus for eliminating or reducing wear and grooving of workpiece carrier retaining elements in order to optimize workpiece throughput rate. Further, there is a need for a method and apparatus for providing improved wafer carrier retaining elements having increased life spans which also function to eliminate or decrease mechanical distortions of the retaining elements which can occur as a result of water absorption. Eliminating or decreasing mechanical distortions of the retaining elements also functions to enhance uniform polishing of workpieces.

SUMMARY OF THE INVENTION The present invention provides a method and apparatus for an improved workpiece retaining structure for use with a workpiece carrier. The improved retaining structure may take the form of a number of prior art embodiments such as, for example, a retaining element in the form of a ring member, wherein the improvement of the present invention lies in the composition of the retaining element. The retaining element is preferably made from a dense, hard ceramic such as, for example, ceramics having a Mohs scale of hardness greater than 7. Such ceramics provide wear resistance from friction created between the wafer and the retaining element, as well as abrasive slurries as such as silicon dioxide, and also eliminating grooving of the retaining element wherein grooves are formed within the retaining element as a result of placing the wafer and retaining element in contact with a polishing pad, rotating the wafer and retaining element, as well as the polishing pad, and moving both the wafer and retaining element relative to the polishing pad.

The retaining structure of the present invention may also include a disposable liner or coating positioned around the inside diameter of the ring member, or retaining ring. The disposable liner functions to prevent the semiconductor wafer from coming into direct contact with the harder ring member, or retaining ring, during polishing and is preferably comprised of a material that is softer than the semiconductor wafer, for example a polymer such as an acetal copolymer or polybutylene terathalate (PBT) in order to prevent the ring member, or retaining ring, from damaging the wafer. The workpiece or semiconductor wafer will tend to abrade and wear out the liner but will not be damaged by the liner.

Examples of hard ceramic materials that may be used in accordance with the method and apparatus of the present invention for eliminating or decreasing wear and grooving of a retaining element include, but are not limited to, Al₂O₃ Be₂C, TiC, SiC, A1B, B₄C and diamond. In addition, the method and apparatus of the present invention is not limited to single-phase materials but may also include combinations of ceramic materials that can be used to enhance the material properties of any retaining element such as, for example, a retaining ring. Such combination ceramic materials may be made of two-phase composite materials such as, for example, aluminum oxide (Al₂O₃) reinforced with silicon carbide (SiC). Silicon carbide whiskers aid to slow or stop crack propagation in the Al₂O₃ matrix thereby toughening the ceramic. The general details of the toughening mechanism arise from the coefficient of thermal expansion (C.T.E.) Mismatch between the SiC and the Al₂O₃. The C.T.E. of SiC is about 4.5 x 10^"6/°C whereas the C.T.E. for Al₂O₃ is approximately 8.6 x 10^"6/°C. Residual stresses from densification at high temperatures results in the SiC whiskers being put into axial compression and the surrounding Al₂O₃ matrix in axial tension. Cracks tend to propagate perpendicular to tensile stresses and are thus attracted to the SiC whiskers. Once a crack intersects a whisker it then propagates parallel or perpendicular to the whisker. The energy of the crack is then turned into work done on the whisker to either fracture or separate the matrix material from the whisker.

Methods for forming retaining ring elements from hard ceramics typically come from pressureless sintering or pressure assisted sintering processes. In pressureless sintering, a homogeneous ceramic slip is prepared which can be organic or aqueous in nature. Once the slip is prepared, it can be spray dried into soft granules for forming. The forming can be done either with a large hydraulic press or isostatically pressed. Machining of the pressed article is commonly performed prior to densification. After forming, the ring must be heated to high temperatures in a furnace. The furnace removes the organic binders in the article and removes residual porosity through various densification mechanisms such as liquid phase assisted material transport, particle rearrangement, and solid state diffusion. Final machining is usually required to bring the ceramic article into proper dimension. In the case of pressure assisted sintering, the same spray dried particles or even raw powder are pressed in a high temperature furnace coupled with a moveable ram to physically press the powder into a solid compact. Similar machining is required to bring the article into final shape.

Advantages of the method and apparatus of the present invention include resulting retaining elements, such as retaining rings, that greatly decrease or eliminate wear and grooving of the retaining elements as well as mechanical distortions of the retaining elements that may result from the retaining elements absorbing water. In addition, other elements such as a disposable liner positioned about the inner diameter of a retaining element, such as a retaining ring, function to prevent the improved retaining ring from damaging the workpiece. Furthermore, the improved retaining structure of the present invention is easy to maintain and requires less downtime for repairs, relative to conventional retaining structures such as retaining rings composed of engineering polymers such as acetal polymers, homopolymers, acetal copolymers and polyphenylene sulfide. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the Figures, where like reference numbers refer to similar elements throughout the Figures, and: FIG. 1 is a side cross-sectional view of one embodiment of a prior art semiconductor wafer carrier element which includes an exemplary embodiment of the retaining structure of the present invention;

FIG. 2 is a top view of the retaining structure for retaining a wafer within the wafer carrier element shown in FIG. 1; FIG. 3a is a schematic showing one means of arresting a crack being formed in a two- phase composite material;

FIG. 3b is a schematic showing another means of arresting a crack being formed in a two-phase composite material; and

FIG. 4 is a flow chart showing the method of the present invention for decreasing or eliminating wear and grooving of workpiece carrier retaining structures used during polishing of workpieces.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT The subject invention relates generally to the polishing of work pieces such as semiconductor wafers. It will be understood, however, that the invention is not limited to a particular workpiece type or to a particular manufacturing or polishing environment.

FIG. 1 depicts a wafer carrier 100 according to one embodiment of the present invention. For the sake of clarity and brevity, wafer carrier 100 is illustrated in a simplistic manner without a number of components that maybe present in a practical carrier. Typically, carrier 100 is mounted at the end of a rotatable and vertically movable drive shaft 102, and above a rotatable polishing surface, e.g., a pad 104, affixed to a platen 105. Wafer earner 100 and the above components are typically integral to a chemical mechanical polishing ("CMP") machine or a similar workpiece polishing apparatus. CMP machines are well known in the art; a detailed description of the construction and operation of an exemplary CMP system may be found in U.S. Pat. No. 5,329,732, issued July 19, 1994 to Karlsrud et al., the disclosure of which is incorporated herein by reference.

Wafer carrier 100 comprises a pressure element 106, a protective wafer backing pad 108, a retaining ring 110, and a disposable liner 112. Pressure element 106 may be rigidly coupled to earner 100 or movably coupled to carrier 100, depending upon the particular configuration of wafer carrier 100. For example, in the illustrated embodiment, pressure element 106 is configured as a rigid pressure plate that is fixed to at least a portion of carrier 100. It should be appreciated that the present invention may be embodied in the context of any number of practical wafer carrier designs, e.g., those utilizing floating pressure plates and gimbal mechanisms, those utilizing fluid driven bladders or membranes instead of rigid pressure plates, those utilizing floating bladder assemblies, and those using any combination of such techniques.

In FIG. 1, pressure element 106 is a unitary component formed of a rigid material, such as steel. Pressure element 106 is configured to hold a workpiece against polishing pad 104 during a polishing operation associated with the CMP system. Wafer carrier 100 may employ any number of known techniques to apply, regulate, and control the amount of pressure imparted by pressure element 106. A compliant wafer backing pad 108 is adhered to the lower surface of pressure element 106 to cushion wafers held thereby and to protect the wafers against damage which may result from direct contact with the pressure element 106. The rear face of the wafer or other workpiece 120 rests in parallel contact against backing pad 108, while the front face of the workpiece 120 is exposed for parallel contact against the top surface of polishing pad 104. The backing pad 108 prevents imperfections or material present on the rear face of the wafer from being "telegraphed" through the wafer to its front (polishing) face, which can result in uneven pressure distribution across the wafer front face against the polishing pad 104 which, in turn, can lead to uneven material removal rates and impaired planarization. The backing pad 108 also frictionally engages the rear surface of the wafer 120, thereby preventing movement or sliding of the wafer 120 relative to the backing pad 108.

Retaining ring 110 is preferably comprised of a hard ceramic such as Al₂O₃, Be₂C, TiC, SiC, A1B, B C, cubic BN, or diamond. In addition, the composition of the retaining ring 110 of the present invention is not limited to a single-phase material but may also be include combinations of ceramic materials such as, for example, a two-phase composite material of Al₂O reinforced with SiC. In addition, disposable liner 112 may be positioned about the interior diameter of the retaining ring 110 to prevent the semiconductor wafer 120 from coming into contact with the retaining ring 110. The disposable liner 112 may also take the form of a coating that is directly applied about the outer circumference of the retaining ring 110 including application over the outer edge of the retaining ring 112. The disposable liner 112 or coating is preferably made of a polymer material that is softer than the semiconductor wafer such as, but not limited to, an acetal copolymer or polybutylene terathalate (PBT).

During the CMP procedure, polishing pad 104 is located below wafer carrier 100 on a rotatable polishing platen 105. The hardness and density of the pad are selected based on the type of material to be planarized. Blown polyurethane pads, such as the IC and GS series of pads available from Rodel Products Corporation of Scottsdale, Arizona, may be advantageously utilized by the CMP system. An abrasive slurry, such as an aqueous slurry of silica particles, is typically pumped onto the polishing pad 104 during a polishing operation. The relative movements of wafer carrier 100 and polishing pad 104, augmented by the abrasive action of the slurry, produce a combined chemical and mechanical process at the exposed (lower) face of a wafer 120 (which is located under pressure element 106) which removes projections and irregularities to produce a substantially flat or planar surface on the lower side of the wafer 120. Wafer 120 may be held against protective wafer backing pad 108 by any convenient mechanism, such as, for example, by vacuum or by wet surface tension. Circular retaining ring 110 and its accompanying disposable liner 112 are preferably connected around the periphery of protective wafer backing pad 108 and prevent wafer 120 from slipping laterally from beneath the protective wafer backing pad 108 as the wafer is polished. Retaining ring 110 is generally connected to pressure plate 12 by bolts 124.

FIG. 2 shows a top view of the exemplary retaining structure 150 for retaining a wafer within the wafer carrier element shown in FIG. 1. The retaining structure 150 comprises a ring member 152 having an inner diameter 154 and an outer diameter 156. The ring member 152 is preferably comprised of a ceramic material having a Mohs scale of hardness greater then 7. Examples of such ceramic materials include, but are not limited to, Al₂O₃ Be₂C, TiC, SiC, AIB, B C, cubic BN, and diamond. In addition, the ceramic material comprising the ring member 152 may comprise more than a one-phase material. For example, the ring member 152 may comprise a two or more phase ceramic material such as but not limited to, the following examples: (i) Al₂O₃ reinforced with SiC to form a two-phase ceramic;

(ii) hot pressed SiC/TiC to form a two-phase ceramic; (iii) liquid phase sintered SiC sintered with Al₂O₃ or Y₂O₃ to form a two- phase ceramic; (iv) liquid phase sintered SiC/TiC sintered with Al₂O₃ or Y₂O₃ to form a three-phase ceramic;

(v) liquid phase-sintered SiC/TiC sintered with Al₂O₃ or Y₂O₃ and coated with chemical vapor deposition SiC coating to form a four-phase ceramic. In addition, a disposable liner or coating 158 may be positioned around the inner diameter 154 of the ring member 152 in order to prevent the ceramic ring member 152 from damaging the wafer during polishing. The disposable liner or coating 158 is preferably made of a material that is softer than the material comprising the workpiece or wafer being polished, e.g., a polymer such as an acetal copolymer, polybutylene terathalate (PBT), or polyimide. Use of the above examples of two or more phase ceramic materials in the method and apparatus of the present invention results in a retaining element, such as a retaining ring, which possesses increased strength and toughness over prior art retaining elements. For example, with respect to example (v) above, the SiC/TiC coefficient of thermal expansion is greater than the outer CVD SiC coating. Accordingly, when the CVD SiC coating is applied at high temperatures, such as 1000°C, the thermal expansion mismatch between the two materials results in placing the CVD-SiC outer coating in compression, thus allowing for in-situ strengthening and toughening of the ceramic. The strength of SiC in compression is typically ten times higher than the strength of SiC in tension. Therefore, placing the outer ceramic coating under compression is important in obtaining a retaining element having increased strength and greater wearability.

Turning now to FIG. 3a, there is shown a means of arresting a crack being formed in a two-phase composite material which is used to form a retaining ring in the improved retaining structure of the present invention. The retaining ring is comprised of an Al₂O₃ matrix 180 which is reinforced with SiC whiskers 182. As a crack 184 develops, and begins to propagate through the Al₂O₃ matrix 180, it encounters a SiC whisker 182. The energy of the crack 184 is then turned into work performed to separate the Al₂O₃ matrix 180 from the SiC whisker 182 as shown by arrows 186.

Another means for arresting a crack forming in an exemplary retaining ring of the present invention formed from a two-phase hard ceramic material is shown in FIG. 3b. In FIG. 3b, a crack 184 develops and begins to propagate in an Al₂O₃ matrix 180 when it encounters a SiC whisker 182. Upon encountering the SiC whisker 182, the energy of the crack 184 is turned into work performed on the SiC whisker 182 to fracture the whisker 182. Once the energy of the crack 184 is used to perform work to fracture the whisker 182, the propagation of the crack 184 is arrested due to lack of energy to continue the propagation of the crack 184. FIG. 4 is a flow chart showing the method of the present invention for decreasing or eliminating wear and grooving of workpiece carrier retaining structures used during polishing of workpieces. First, a homogenous ceramic slip is formed 200 from a ceramic material. Next, the ceramic slip is spray dried 202 into soft granules for forming. The soft granules are then formed by applying a hydraulic press or isostatic pressure 204. In the case of pressureless sintering, the pressed article, such as a ring member, is heated to high temperatures 206 in a furnace. Alternatively, in the case of pressure assisted sintering, the soft granules are heated in a high temperature furnace and pressed with a moveable ram 208 to physically press the particles into a solid compact. During the heating step, organic binders are removed from the ring member and residual porosity is also removed through various densification mechanisms such as liquid phase assisted material transport, particle rearrangement, and sold state diffusion. Final machining 210 of the ceramic article, or ceramic ring member, is usually required to bring the ceramic article into proper dimension. Next, a disposable liner is optionally positioned or coated about the inner diameter of the ceramic ring member 212 in order to prevent the ceramic ring member from damaging the workpiece during polishing. The ceramic ring member and liner are then positioned in the workpiece or wafer carrier 214 and the ceramic ring member and liner are then pressed against a polishing surface 216 during polishing of the workpiece or wafer. As a result a method and apparatus is established for eliminating wear and grooving of workpiece carrier retaining structures such as retaining rings during workpiece polishing.

The present invention has been described above with reference to exemplary embodiments. However, those skilled in the art will recognize that changes and modifications may be made to the exemplary embodiments without departing from the scope of the present invention. For example, the ceramic retaining ring of the present invention may be comprised of multiple-phase ceramic materials in addition to those specific examples given as long as they enhance the material properties of the retaining ring such that wear and grooving of the retaining ring is reduced or eliminated. These and other changes or modifications are intended to be included within the scope of the present invention, as expressed in the following claims.

Claims

CLAIMSWhat is claimed is:

1. A workpiece carrier retaining structure comprising a ring member wherein said ring member is comprised of a composite ceramic material formed in two or more phases.

2. The workpiece carrier retaining structure of claim 1 wherein said two-phase composite ceramic has a Mohs scale of hardness greater than 7.

3. The workpiece carrier retaining structure of claim 2 wherein said first and second ceramic materials each comprise at least one of Al₂O₃, ZrB₂, Be₂C, TiC, SiC, AIB, B C, cubic BN, and diamond.

4. The workpiece carrier retaining structure of claim 3 wherein said composite ceramic comprises liquid phase sintered SiC/TiC coated with a CVD SiC coating.

5. The workpiece carrier of claim 1 wherein said composite ceramic material is capable of absorbing water to eliminate mechanical distortions occurring in said ring member.

6. The workpiece carrier retaining structure of claim 1 further comprising at least one of a protective liner or a protective coating positioned about said ring member such that said ring member does not come into direct contact with a workpiece during polishing.

7. A workpiece carrier for use with a workpiece polishing system, said workpiece carrier comprising: a carrier housing having an upper end and a lower end; a pressure element operatively associated with said carrier housing and being configured to hold a workpiece against a polishing surface during a polishing operation associated with said workpiece polishing system; and a workpiece retaining assembly, said workpiece retaining assembly comprising at least one retaining element wherein said at least one retaining element is comprised of a composite ceramic material having two or more phases.

8. A workpiece carrier according to claim 7 wherein said at least one retaining element is comprised of a two or more phase ceramic material having a Mohs scale of hardness greater than 7.

9. A workpiece carrier according to claim 8 wherein said composite ceramic material comprises two or more ceramics each comprising at least one of Al₂O₃, ZrB₂, Be₂C, TiC, SiC,

AIB, B₄C, cubic BN, and diamond.

10. A workpiece carrier according to claim 7 wherein said two or more phase composite ceramic material comprises one ceramic material reinforced with whiskers of another ceramic material.

11. A workpiece carrier according to claim 7 wherein said at least one retaining element comprises a ring member.

12. A workpiece according to claim 7 wherein said composite ceramic material is capable of absorbing water to eliminate mechanical distortions occurring in said at least one retaining element.

13. A workpiece according to claim 7 wherein said two or more phase composite ceramic material comprises liquid phase sintered SiC/TiC coated with a CVD SiC coating.

14. A method for making a workpiece retaining element used to retain a workpiece during polishing comprising the step of forming said retaining element from a two or more phase composite ceramic material.

15. The method of claim 14 wherein said step of forming said retaining element from at least one ceramic material comprises the step of forming said retaining element from at least one of Al₂O₃, ZrB₂, Be₂C, TiC, SiC, AIB, B C, cubic BN, and diamond.

16. The method of claim 14 wherein said step of forming said retaining element from at least one ceramic further comprises the step of forming said retaining element from at least a two-phase composite material.

17. The method of claim 14 wherein said step of forming said retaining element from at least a two-phase composite material comprises the step of forming said at least two- phase composite material from liquid phase sintered SiC/TiC.

18. The method of claim 17 further comprising the step of coating said retaining element with CVD SiC coating.

19. The method of claim 16 wherein said step of forming said retaining element from at least a two-phase ceramic composite material comprises the steps of:

(a) forming a retaining element from a first ceramic material; and (b) applying a second ceramic material to said first ceramic material such that said second ceramic material is in compression.